Chapter Four: Nature and reflection of light

LIGHT

Is an invisible form of energy that causes the sensation of vision in

us through eyes

Sources of Light

Sources of light is the original of light in which the light are

comes from whether natural or artificial

Types of Sources of Light

• Natural sources of light. For example, sun, star and lighting

• Artificial sources of light. For example, torch, candle, kerosene

lamp etc

Properties of Light

• Light radiates (spread out) from its

source

• Light travels in straight line

• Light transfers energy.

• Light travels in vacuum

• Light travels at the fast speed, about

300,000,000m/s (300,000 km/s)

NB:

❖ All objects which give out their own light are called Luminous

Objects. e.g. star, sun, torch, candle, electric bulb etc

❖ All objects that do not emit their own light instead became visible

when they reflect light from another source are called Non-

Luminous Objects. E.g. moon

❖ All objects that emit light as a result of being heated are called

Incandescent

Objects. e.g. light bulb, fire flame, candle flame etc

❖ The spreading of light from its source to the environment in

straight lines is referred as Rectilinear Propagation of Light

Propagation of Light

 Light travels in a straight line

Ray

• Ray is the path travelled by light.

• Ray is represented in a diagram by full straight line with an arrow

to show the direction of light.

Beam

 Beam is a collection of rays of light

Page

Types of Rays

• Parallel rays

• Converging rays

• Diverging rays

Parallel Rays

 The collection of rays in a straight line which can never cross each

other

Converging Rays

 The collection of rays to one point

Diverging Rays

 The spreading out of rays from one point

Transmission of Light

Bodies (objects) can be grouped according to transmission of light through

them such as: - a) Opaque bodies

b) Translucent bodies

c) Transparent bodies

Opaque bodies

•

Are the bodies which do not allow light to pass through them.

For example, stone, wood, concrete walls, books etc

Translucent Objects

• Are the objects which allow small amount of light to pass through

them.

• For example, oiled paper, tinted glass, some plastic materials etc

Transparent Objects

• Are the bodies which allow all light to pass through them.

• For example, glass, pure water, air etc

Shadow

 Is a dark area where light from a light source is blocked by an

opaque object

Types of shadow

• Umbra shadow

• Penumbra shadow

Umbra Shadow

• Is the total shadow formed behind the opaque bodies.

• It receives no light at all from the source.

Penumbra Shadow

• Is the partial shadow formed behind the opaque bodies.

• It receives some light from the source

NB:

• When source of light is small than opaque only umbra is formed

Eclipse

 Is the shadowing or shading of one heavenly body in the shadow

of another.

Types of Eclipse

• Solar eclipse

• Lunar eclipse

Solar Eclipse

• Is the kind of eclipse in which the moon is between the earth and

the sun.

• Always occurs during the day.

• The area covered by the shadow is the umbra in which the sun cannot

be seen at all

Lunar Eclipse

 Is the kind of eclipse in which the earth is between the sun and the

moon and the shadow of the earth is cast on the moon.

Reflection of Light

 Is the bouncing back of light rays when they meet an obstacle in

their path

Terms used

• Incident ray is the ray of light which strikes a surface

• Reflected ray is the ray that represents the light reflected by the

surface

Types of Reflection

• Regular reflection

• Diffuse reflection

Regular Reflection

 Is the reflection where by all reflected rays reflected in one

direction.

 The rays are in parallel to each other. Occurs at smooth

surface

Diffuse (Irregular) Reflection

• Is the reflection where by all reflected rays reflected random or in

different directions

• It Occurs at a rough surface

NB:

• We can see our images clear in a plane mirror as a result of

regular reflection.

• If light falls in polished surface at right angle, it is reflected back

into the air on the same pass

 Diffuse reflection also called scattering/ irregular reflection

• When the sun rays enter the earth’s atmosphere, it begins to be

scattered by molecules of nitrogen and oxygen.

• The sky looks blue on a clear sunny day because these molecules

scatter the blue light more than other colors due to its shorter

wavelength

Laws of Reflection

Consider the figure below

From the figure above

• ON is a perpendicular line to the surface of the mirror (It is called

the Normal)

• Normal is the line which divides the angle of incidence and angle

of reflection into two equal angles

• Angle of incidence (i) is the angle between the incident ray and

the normal

• Angle of reflection (r) is the angle between the reflected ray and

the normal

• Thus, the laws of reflection states that

1^st. “The incident ray, the reflected ray and the normal all lie in

the same plane”

2^nd“The angle of incidence equals to the angle of

reflection” (i = r) Images Formed by Plane Mirrors

• When an object is kept in front of plane mirror the image is formed

due to the reflection of light

Whereby:

M = magnification

Id = image distance

Od = object distance

IH = image height

OH = object height

Characteristics of Image formed in a Plane Mirror

✓ The image is virtual (not real)

✓ The image is upright

✓ The Image and object have the same size

✓ The image distance is the same as the object distance from the

plane mirror

✓ The image has a left-right reversal (laterally inverted)

Magnification

 Magnification is given by the formula

Rotating a mirror

• The reflected ray moves through an angle twice the angle of rotation

• If the mirror was rotated through certain angle θ, then the reflected ray

would be rotated through an angle of 2θ

Multiple Mirrors

• Is the system which consists of two or more mirrors and produce

several images of the same object.

• Right – angle mirrors refer to two mirrors that are joined at their

edges at an angle of 90⁰

• Image in parallel mirrors (two mirrors joined at 0⁰). The image

formed is at infinite in each mirror because there is a repetition

of images

NB:

▪ The number of images increase as if the angle between the

mirrors decreases

i.e

▪ Parallel mirrors are commonly used in saloons and barber shops

▪ The number of images (푛) formed between mirrors placed at 휽⁰,

is given by the formula:

Application of Reflection of Light

 It is applied in Periscope

 Periscope is a device used for seeing objects that are above the

eye level of the observer